The present invention relates to the production of laminated structures of thermoplastic resinous materials, and more especially, to a process for the production of multiple-layered laminates having at least three layers of different thermoplastic synthetic resinous material.
In recent years the use of laminates constructed from two or more layers of different thermoplastic synthetic resinous materials has gained widespread acceptance in the packaging and container field. Typically, an individual laminated product is tailored specifically to the end use contemplated for the material, and the thermoplastic materials utilized for each of the respective layers of the laminate are chosen accordingly. Hence, in some instances a good heat sealing material, such as polyethylene, may be chosen for one or more surface layers of a laminated product which requires heat sealing for closure, while in other instances polymers having excellent chemical resistance properties, such as acrylic ester polymers or fluorine containing polymers, may be chosen as surface layer materials, and in still other areas of utility a thermoplastic material having desirable barrier properties, e.g., saran, may be chosen for one or more of the individual layers of a laminated product.
The manner and ease of producing laminates of the foregoing type depends upon the particular polymeric materials chosen to construct the laminate and, to some extent, also upon the configuration of these materials in the final laminated product. It is possible to produce most laminated structures by independently forming each layer, for example, by extrusion, and then joining the individual layers either immediately as they exit from the extrusion equipment in a heat-plastified state or by joining such pre-formed materials under the influence of heat and pressure. Proceeding according to this method, however, usually requires a large investment in equipment and oftentimes results in an unsatisfactory product, e.g., inadequate bonding of layers.
On the other hand, many types of thermoplastic synthetic resinous materials, although, they possess excellent chemical and/or physical properties, cannot be made to readily or securely bond to other types of polymeric material with which it may be desired to combine them to form multiple layer laminated materials. Under these circumstances, it is necessary to interpose a layer of an adhesive material between the respective layers of polymeric material in order to produce a satisfactory laminate, and to accomplish this by conventional laminating techniques usually requires additional equipment or very complex and costly equipment designed specifically to accomplish this result. See, for example, U.S. Pat. No. 3,184,358.
Although the use of laminates constructed from two or more layers of different thermoplastic synthetic resinous materials has gained widespread acceptance in the packaging and container field in recent years, thermoformed and particularly deep drawn thermoformed products have, however, remained basically single layer materials, principally high impact polystyrene. The reason for this is that high impact polystyrene is relatively inexpensive, is easy to thermoform, and has good structural strength characteristics. However, polystyrene is fairly permeable by gases. This makes it unacceptable for packaging spoilable food items, carbonated beverages, etc. For this reason it has previously been suggested that there be added a layer having high barrier characteristics such as polyvinyl chloride (PVC), polyvinylidene chloride (PVDC) or copolymers thereof such as saran. See, for example, U.S. Pat. Nos. 3,798,103 to Gaunt; 3,654,069 to Freudenberg; 3,458,392 to Kremer and 3,328,196 to Sincock.
As mentioned in these patents, however, lamination of a barrier layer to polystyrene is difficult to obtain. Therefore, special procedures and/or adhesives have been used. Thus, Sincock discloses using a solvent coated polymethacrylate adhesive as a subcoat beneath a saran film or saran emulsion coating. Kremer solvent coats a combination wood rosin and liquid polystyrene adhesive as the bonding agent between layers of polystyrene and saran. Others, such as Newman in U.S. Pat. Nos. 3,645,538 and Lee in 3,415,920, have found it necessary to place the saran interiorly as a middle layer between polystyrene or other plastic layers with an adhesive layer on each side of the saran. It is suggested in this latter patent to employ as an adhesion-promoting agent for bonding the outer polyolefin layers to the centrally disposed barrier layer polymeric materials such as copolymers of chlorinated ethylene and ethyl acrylate, copolymers of ethylene and vinyl acetate and chlorinated polyethylene having from about 20 to 40 weight percent chlorine.
Placing the barrier material layer on the interior of the laminate represents a significant disadvantage, because it means that a layer of non-barrier material must be interposed between the barrier material layer and the package contents. This layer of non-barrier material gives rise to a wicking action which negates the effectiveness of the barrier layer. Moreover, the non-barrier material may stress crack and destroy the functionality of the entire laminate.
However, with such a crystalline material as a barrier layer in the laminate, it is difficult to thermoform without blistering and delamination occurring during the drawing operation. Generally, the strength of the lamination bond controls this feature. The adhesive strength also determines the amount of delamination later suffered when handling, or peeling off lids, labels, or like attachments to the laminated container. Similarly, the adhesive strength of the bond and crystalline nature of the barrier layer have other effects on the deep-draw characteristics of the laminate. Many such laminates cannot be thermoformed adequately since at the desired lower temperatures which will produce an orientation of the crystalline layer, the lamina will not stretch adequately, but will fracture and delaminate. On the other hand, if thermoformed at a higher temperature, the stretching will not produce an oriented barrier film. In either case, the resulting features such as water-vapor transmission rate (WVTR) and oxygen transmission rate (OTR) will not be adequate for many container packaging purposes.
The type of prior art laminates discussed above suffer from one or more of these defects. In addition, those that utilize solvent based adhesives or emulsion coatings require high capitalization costs due to the types of laminators, coaters, drying ovens, etc., required. There is also considerable waste in all of the aforementioned processes in that the trimmed materials are not as recoverable in adhesive lamination operations as, for example, in extrusion processes.
With the advent of coextrusion processes, it is now possible to coextrude many combinations of thermoplastic synthetic resinous materials which yield multiple-layer laminates having desirable properties and characteristics. See, for example, British Pat. No. 1,364,176 which discloses coextrusion of high impact polystyrene and acrylonitrile-butadiene/styrene (ABS); U.S. Pat. No. 3,746,609 which discloses coextrusion of a polystyrene material and a polyamide; U.S. Pat. No. 3,798,103 which discloses coextrusion of high impact polystyrene and polyvinyl chloride or Barex; and U.S. Pat. No. 3,654,069 which discloses as one embodiment coextrusion of high impact polystyrene and styrene-acrylonitrile copolymers to which a polymethylmethacrylate film may be laminated.
Still with other types of thermoplastic synthetic resins such as barrier materials, like saran, special considerations are presented insofar as processing by extrusion or coextrusion is concerned as a result of their corrosive nature and other unusual properties, so that coextrusion of these materials has been severely limited. Thus, the use of these polymeric materials in multiple-layered laminates has been solely restricted to the solvent-based adhesive layer or inner saran layer arrangements previously discussed. This is despite the fact that it is desirable to have an outer barrier layer which can serve as the interior layer of a food container, which can be sterilized and which is attractive. It has not been possible to successfully coextrude a saran-polystyrene laminated material with a saran exterior layer. Accordingly, the limitations of poor adhesion and processing difficulties with saran have resulted in the inability to conveniently produce a suitable laminate of polystyrene and saran wherein the saran occupies the position of at least one outside layer of the laminate. A convenient method of producing such a product would be highly desirable.